1. Field of the Invention
The present invention relates to a media thickness detector, and more particularly, to a media thickness detector capable of detecting the thickness of media to be fed.
2. Description of the Related Art
A media thickness detector detects the thickness of media, such as bank notes, checks, tickets, certificates and paper. The reason for detecting the thickness of media is to ensure smooth operations of an apparatus by preventing a plurality of sheets of media from being fed together or by preventing inferior media from being fed. For example, such a media thickness detector is used in automatic teller machines, ticket-issuing machines, certificate-issuing machines, printers, facsimile machines, and the like.
Methods of detecting the thickness of media include a method of detecting the thickness of media in which a roller comes into direct contact with media by means of a mechanical mechanism, a method of detecting the thickness of media using an optical sensor without bringing a mechanical mechanism into direct contact with media, and a combination thereof
FIGS. 1 and 2 show a media thickness detector in accordance with the related art. Referring to these figures, a sensing roller 12 is installed on a rotational shaft 10 of which both ends are supported on a frame (not shown). The sensing roller 12 is disposed on a path along which media m travel. The sensing roller 12 is rotated by a driving belt 14 that is driven by a driving source. The driving belt 14 connects the driving source and the rotational shaft 10 to transmit power from the driving source to the rotational shaft 10, thereby causing the sensing roller 12 to rotate. A timing belt is used for the driving belt 14.
A central shaft 16 of which both ends are supported on the flame is provided in the vicinity of the rotational shaft 10. A bearing holder 18 is installed on the central shaft 16. Return springs 19 are installed at both ends of the central shaft 16 provided with the bearing holder 18. An end of each of the return springs 19 is connected to the bearing holding 18 at a point spaced from the central shaft 16 and the other end of the return spring 19 is connected to the frame, whereby the return spring imparts a restoring force in a specific direction. Therefore, the bearing holder 18 tends to always rotate in a specific direction.
Sensing bearings 20 are provided at a side of the bearing holder 18. As seen from FIG. 2, the two sensing bearings 20 are provided at both ends of the one side of the bearing holder 18, respectively. The circumference of the sensing bearing 20 is set to be approximately a half of the traveling length of media m.
The sensing bearings 20 are installed on the bearing holder 18 such that they can be freely rotated. The sensing bearings 20 tend to always move in a direction in which they are brought into close contact with the sensing roller 12, because the bearing holder 18 is rotated toward the sensing roller 12 by means of the restoring force of the return springs 19. The media m to have its thickness detected is fed between the sensing roller 12 and the sensing bearings 20.
A thickness sensor 22 detects the thickness of media m by detecting a rotating amount of the bearing holder 18. An example of the thickness sensor 22 includes a RVDT (Rotational Variable Differential Transformer) sensor.
However, the aforementioned related art has the following problems.
First, since the sensing roller 12 is rotated by receiving a driving force from the driving source through the driving belt 14, vibration from the driving source is transmitted to the sensing roller 12 through the driving belt 14. When vibration is transmitted from the driving source to the sensing roller 12, there is a problem that the thickness of media m cannot be accurately measured, particularly when a mechanical mechanism is used for detecting the thickness.
Further, each of the sensing bearings 20 must come into contact with the media. However, the two sensing bearings 20 are installed on a single bearing holder 18. Therefore, the sensing bearings 20 cannot be brought into close contact with the sensing roller 12 with the same force when the bearing holder 18 is not accurately installed. In other words, if the bearing holder 18 is slightly tilted because of manufacturing tolerances, one sensing bearing 20 will bear against the media with a greater force than the other sensing media. Accordingly, there is also a problem in that the thickness of media m cannot be accurately detected.
In addition, since the circumference or perimeter length of the sensing bearing 20 is set to be about a half of the traveling length of media m, the sensing bearing 20 is rotated twice while a sheet of media m travels between the sensing bearing and the sensing roller. Therefore, there is another problem in that the thickness detection capability is deteriorated since measuring errors that may be produced due to the tolerance of the sensing bearing 20 are accumulated.